2000 Year Old Date Seeds Still Viable

Here's a little snippet that you can lob in at your next dinner party conversation with those who are interested in growing things.

Flowering plant seeds can be broadly classified as (i) orthodox, which have desiccation tolerance and can be stored for varying periods without unduly impacting viability and the ability to germinate, and (ii) recalcitrant (many tropical species), which are best kept close to the moist state existing within ripe fruit and sown as quickly as possible without any significant post-harvest drying. There are some overlaps between these two categories and some researchers include an intermediate category, stressing that variations between all three really manifest as a continuum. But if we stay with the conventional grouping, many seeds of sub-tropical fruiting species are orthodox, and with appropriate preparation and care this allows growers to vary the time between the time that seed are obtained and sowing, sometimes for several years while still producing good germination outcomes. Seed suitable for storing is dried, often down to the 5% moisture level, and optimal storage life is achieved if the low hydration level is continuously maintained. Imbibition when water is available begins the rehydration process leading to activation of enzymes and other processes associated with germination (providing there is no endo-dormancy), and this will reduce subsequent storage lifetime. Low temperature is another beneficial storage factor as it slows down metabolism that would otherwise use up finite energy reserves and could generate damaging chemical species or reactions that may not be effectively inactivated or repaired. Other storage factors beyond moisture and temperature include differences in species genetics eg seed coat thickness and permeability, internal antioxidant levels and repair mechanisms, and management differences such as maturity of seed at harvest and the rate of subsequent desiccation, predation, pathogens or wounding. Low humidity and temperature conditions are the basis of international seed storage banks.

Under favourable conditions, how long can orthodox seed storage life be extended? There are many instances of seeds being kept for several years that are still in good condition, but most would find it hard to imagine it could go on for too much longer. Well, a 1960s excavation near the Dead Sea in Israel uncovered some ancient materials that were subsequently identified as date palm seeds (Phoenix dactylifera, Science (2008) 320, 1464). They had been kept in storage at room temperature for 40 years when 5 seeds were selected to determine their radio carbon age and to see if any were still viable and would germinate. Two were used for the dating which was found to be 50 yrs BC, ie more than 2000 years old, and one of the other 3 germinated, with the seedling growing to 15, 31 and 121cm in 3, 7.5 and 26 mths resp. Genetic analysis suggested this plant had many commonalities with modern elite Egyptian, Iraqi and Moroccan varieties, and the researchers suggested that local high summer temperatures and the dry climate contributed to the extended seed longevity.

But dates are not the runaway longevity winner if we branch outside natural seed reproduction to what can be achieved in the laboratory from other ancient fruits. In a 2012 study (Proc Nat Acad Sci, 109, 4008-4013), Russian scientists found that immature placental tissue of the herbaceous perennial Silene linnaeana buried in Siberian permafrost squirrel caches, carbon dated at 31,800 years old, was able to be micro-propagated and then grown on to become fertile adult plants producing flowers and fruit. Seeds taken from the fruits of micro-propagated plants germinated normally, and these seedlings grew on to exhibit normal flowering and fruit set. This spectacular longevity had been achieved by cryopreservation 38m underground where the present day average annual temperature is -7°C at ground level.